Thermally stable high molecular weight acyl peroxycarbonic esters in polyester curing

ABSTRACT

Acyl peroxycarbonic esters of the formula ##STR1## in which R is methyl or ethyl, R&#39; is alkyl, and R and R&#39; together have 17 or more carbon atoms. The compounds have unusual thermal stability, together with improved polymerization initiating efficiency. Particularly outstanding efficiency is found in connection with the curing of polyester resins.

This application is a continuation-in-part of patent application Ser. No. 696,463 filed June 16, 1976 now U.S. Pat. No. 4,051,310 for "Thermally Stable High Molecular Weight Acyl Peroxycarbonic Esters."

This invention relates to organic peroxide compositions and their use as polymerization initiators. More particularly, the invention relates to certain higher molecular weight acyl peroxycarbonic esters that exhibit improved thermal stability, together with increased polymerization initiation efficiencies and other improved properties.

U.S. Pat. No. 3,108,093 describes a number of peroxycarbonic esters including a group of acyl peroxycarbonic esters. The molecules disclosed therein are stated to have thermal stabilities which do not "complicate the polymerization process they catalyze." The patentees, this, recognize the potential problem of thermal stability in molecules of this type.

U.S. Pat. Nos. 3,585,232, 3,652,524 and 3,736,344 describe a group of acyl peroxycarbonic esters having lower molecular weights than described in said U.S. Pat. No. 3,108,093 and which have improved efficiencies in the polymerization of monomers.

The present invention provides a group of related acyl peroxycarbonic esters which demonstrate dramatically improved thermal stabilities as compared with analogous molecules described in U.S. Pat. No. 3,108,093. Further, in addition to being more thermally stable, they exhibit improvements in the polymerization of certain compounds, including ethylene, styrene, vinyl chloride, vinyl chloride and vinyl acetate comonomers, methyl methacrylate, in the curing of ethylenically unsaturated polyester with cross-linkable monomer, and in the vulcanization of general purpose silicone rubber (silicone rubber that is cross-linkable through ethylenically unsaturated sites). Particularly outstanding improvements have been found in the polymerization of methyl methacrylate. Also, in the curing the ethylenically unsaturated polyester in the presence of tertiary amine promoters it has been found that the present initiators produce less yellowing in the polymeric end product.

These improvements are obtained in the present invention through the use of acyl peroxycarbonic esters of the formula: ##STR2## where R is selected from methyl and ethyl; and R' is an alkyl group which together with R contains a total of 17 to about 25 carbon atoms. Preferably, R and R' have a total of 17 to about 20 carbon atoms.

Apart from the employment of the novel compounds of this invention, the polymerization and vulcanization processes in which they are used is consistent with the teachings of the prior art, including quantities of reactants, temperatures, solvents, optional additives and the like.

The unsaturated polyester resins cured by the present process comprise a linear or only slightly branched polyester resin and a peroxide cross-linkable monomeric compound. The linear or slightly branched polyester resin is typically prepared as a condensation or reaction product of an unsaturated polybasic and a polyhydric compound; for example, the condensation product of an unsaturated dibasic acid of alpha-beta ethylenic unsaturation and a di or trihydric compound, such as a glycol. Often a saturated polybasic acid or anhydride, such as a dibasic acid, is employed with the unsaturated acid or anhydride to modify the reactivity of the unsaturated resin.

Examples of typical polyhydric alcohols include, but are not limited to: ethylene glycol; 1,2-propane diol; 1,3-propane diol; diethylene glycol; dipropylene glycol; triethylene glycol; tripropylene glycol; 1,2-butane diol; 1,3-butane diol; 1,4-butane diol; neopentyl glycol; 2,2,5-trimethylpentane diol; cyclohexanedimethanol; dibromoneopentyl glycol; dibromobutane diol; trimethylolpropane; pentaerythritol; trimethylpentane diol; dipropoxy adducts of bis phenol A; and dipropoxy adducts of hydrogenated bis phenol A.

Examples of saturated polybasic acids include, but are not limited to: isophthalic acid; orthophthalic acid; terephthalic acid; tetrabromophthalic acid; tetrachlorophthalic acid; tetrahydrophthalic acid; adipic acid; succinic acid; azelaic acid; glutaric acid; nadic acid and the various anhydrides obtained therefrom.

Examples of unsaturated polybasic acids include, but are not limited to: maleic acid; fumaric acid; itaconic acid; citraconic acid and anhydrides obtained therefrom.

Examples of peroxide curable cross-linking monomers employed with the linear polyesters include, but are not limited to: styrene; vinyl toluene; acrylates and methacrylates like methylmethacrylate; alphamethyl styrene; chloro styrene; and diallyl phthalate. The liquid unsaturated polyester resins also typically contain small amounts of inhibitors in order to prevent premature reaction, such as, for example: hydroquinone; quinone and tertiary butyl catechol. These monomers, the saturated acids, the unsaturated acids and the polyhydric compounds may be admixed together in various proportions as is known in the art in order to obtain resins with varying properties, typically in amounts of about 0 to 50% by weight; for example, such as 5 to 45%. Such liquid resin compositions may include a wide variety of other additives to include: viscosity index improvers; rheological agents; flame retardants; thermoplastic polymers; fillers such as hollow glass or plastic microsphere beads; wood flour; silica; diatomaceous earth; pigments; dyes; stabilizers; glass fibers; release agents; extenders; catalysts; alumina surfactants; and other additives (see, for example, compounds in "Unsaturated Polyester", Modern Plastics Encyclopedia, Volume 50, No. 10a, 1973-1974, pp. 66-68, hereby incorporated by reference).

The components of the polyester resins may be varied as is known in the art to impart the desired properties to the cured resin. Typically, flexible resins employ greater amounts of adipates or azeleates, while more rigid resins use phthalates, both with a variety of different glycols.

The following example which is for the preparation of acetyl peroxystearyl carbonate illustrates the procedure utilized to prepare the various acyl peroxycarbonic esters listed in Table I. The other novel compounds of this invention were prepared by substitution of the appropriate chloroformate and peracid in the following example.

To 9.34 g. (0.0295 mole) of 24% peracetic acid in ethyl acetate and 25 ml acetone containing 1 drop of Triton X-100 surfactant is added simultaneously 8.94 g. (0.0268 mole) of stearyl chloroformate and 2.76 g. (0.0349 mole) of pyridine over a 17-minute period at -3° to -5° C. The addition funnel was rinsed with 7-8 ml of hexane. The reaction mixture is well stirred during these additions. After the reaction mixture is stirred an additional 2 hours at -5° C., the reaction mixture is filtered to remove the pyridinium hydrochloride. The filter cake and reactor were rinsed with ether. The cake was transferred to a separatory funnel and extracted with 50 ml CHCl₃ and then with 50 ml ether. These two extractions were combined with the initial filtrate in another separatory funnel. The organic layer was washed as follows: once with 100 ml saturated NaCl solution, pH 6-7; once with 100 ml cold 1% H₂ SO₄ solution, pH 2; once with 100 ml saturated NaCl solution, pH 2; once with 100 ml saturated NaHCO₃ solution, pH 8; once with 100 ml ice water, pH 8; and finally twice with 100 ml saturated NaCl solution, both pH's 7. The organic layer was filtered through a cake of anhydrous MgSO₄ and concentrated under vacuum on a 10° C. H₂ O bath. Product A.O. (active oxygen) analysis: Theory, 4.29; found 3.79, 88.45% pure; 81.1% yield.

                                      TABLE I                                      __________________________________________________________________________      ##STR3##                                                                                     R + R'                Ten Hour, Half-Life.sub.1)                                                                   Synthesis                                                                             Results              Peroxide       (No. of C Atoms)                                                                         % Purity                                                                            m.p. ° C                                                                       Temp, ° C.                                                                            % Purity                                                                              %                    __________________________________________________________________________                                                               Yield                  Acetyl peroxycetyl                                                                          17        96.3 44-45  55.1          94.12  87.7                   carbonate (APCC)                                                               Acetyl peroxystearyl                                                                        19        97.7 54-55  54.9          88.45  81.1                   carbonate (APSC)                                                               Propionyl peroxycetyl                                                                       18        96.0 30-31  51.5          88.72  85.0                   carbonate (PPCC)                                                               Propionyl peroxystearyl                                                                     20        96.0 39-40  52.3          87.87  84.4                   carbonate (PPSC)                                                               Lauroyl peroxyoctyl                                                                         19        98.7 4-6    51.4          87.58  88.7                   carbonate (LPOC)                                                             __________________________________________________________________________      +HD 1) 0.2 M peroxide in benzene                                         

The storage stability of the new molecules of this invention, including the improved thermal stability at 30° C. is shown in the following Table II. The abbreviations of the molecule names are the same throughout and are defined in Table I. LPOC is included as typical of the prior art as disclosed in U.S. Pat. No. 3,108,093.

                                      TABLE II                                     __________________________________________________________________________     Storage Stability of Various Acyl Peroxycarbonic Esters at 30° C        Peroxide                                                                               APSC      APCC     PPSC     PPCC     LPOC                              T A O.sup.1)                                                                           4.29      4.64     4.14     4.46     4.29                              Days Elapsed                                                                           A.O..sup.2)                                                                         % Purity                                                                            A.O.                                                                               % Purity                                                                            A.O.                                                                               % Purity                                                                            A.O.                                                                               % Purity                                                                            A.O.                                                                               % purity                      __________________________________________________________________________      0      4.19 97.67                                                                               4.55                                                                               98.06                                                                               3.90                                                                               94.20                                                                               4.36                                                                               97.76                                                                               4.22                                                                               98.37                          1      --   --   --  --   --  --   --  --   3.57                                                                               83.22                          4      --   --   --  --   --  --   --  --   2.04                                                                               47.55                          6      --   --   --  --   --  --   --  --   1.52                                                                               35.43                          7      --   --   --  --   --  --   1.95                                                                               43.74                                                                               --  --                             8      --   --   --  --   --  --   --  --   1.19                                                                               27.74                         11      --   --   --  --   --  --   1.27                                                                               28.48                                  13      --   --   --  --   3.84                                                                               92.75                                           25      4.16 96.97                                                                               4.55                                                                               98.06                                                                               --  --                                              28      --   --   --  --   3.58                                                                               86.47                                           32      --   --   --  --   3.17                                                                               76.57                                           39      4.14 96.50                                                                               4.57                                                                               98.49                                                                               --  --                                              46      --   --   --  --   1.07                                                                               25.85                                           52      4.12 96.00                                                                               --  --                                                       53      --   --   4.57                                                                               98.49                                                    80      4.07 94.86                                                                               --  --                                                       81      --   --   4.52                                                                               97.41                                                    95      --   --   4.51                                                                               97.20                                                    97      4.10 95.57                                                                               --  --                                                       __________________________________________________________________________      +hu 1) Theoretical Active Oxygen                                               .sup.2) Active Oxygen                                                    

In view of the extroaordinary stability of APSC and APCC, these two molecules were further tested at an even higher temperature of 41-42° C. for a period of time. The results of this additional test are shown in Table II-A below.

                  TABLE II-A                                                       ______________________________________                                         Storage Stability of Two Acyl Peroxycarbonic                                   Esters at 41-42° C.                                                     Peroxide   APSC           APCC                                                 TAO        4.29           4.64                                                 Days Elapsed                                                                              A.O.     % Purity  A.O.   % Purity                                  ______________________________________                                          0         4.22     98.37     4.60   99.14                                      3         4.22     98.37     4.59   98.92                                      9         4.22     98.37     4.47   96.34                                     15         4.20     97.90     2.57   55.39                                     17         --       --        1.03   22.20                                     20         4.25     99.07                                                      28         4.22     98.37                                                      35         4.15     96.74                                                      42         4.16     96.97                                                      ______________________________________                                    

The compounds of this invention are relatively safely handled peroxides as shown by OPPSD tests (standard safety tests of Society Of The Plastics Industry, Inc., Organic Peroxide Producers Safety Division) as set forth in Table III below.

                                      TABLE III                                    __________________________________________________________________________     OPPSD Tests Made on Various Acyl Peroxycarbonic                                Esters (99% Purity)                                                            Peroxide            APSC   APCC   PPSC                                         __________________________________________________________________________       Rapid Heat                                                                            Decomp. Temp, ° C                                                                  82     77     85                                                    Force      Inter.sup.1)                                                                          Inter  Inter                                          PVT    Vent Diameter, mm                                                                         <11/2  2      <1                                                    Hazard     Low-Inter                                                                             Inter  Low                                            Burning                                                                               Flame Height, cm                                                                          46     122    137                                                   Hazard     Low    Inter-Max                                                                             Inter-Max                                      Autoignition at 330-340° C                                                                None   None   None                                           SADT (Estimated) for 7#                                                                          55° C                                                                          46° C                                                                          43° C                                   in 4 gal Fiberpak                                                            __________________________________________________________________________      +hu 1) Intermediate                                                      

The utility and improved efficiencies of the present molecules in polymerization reactions is demonstrated by the following examples in connection with a series of different polymerizable and vulcanizable materials.

Polyester Resins

Data on the testing of these new acyl peroxycarbonic esters for the curing of ethylenically unsaturated polyester resions are found in Tables IV-VI.

In Table IV, results of testing a series of peroxides at equivalent A.O. in a Hot Block Gel Tester at 180° F. are shown. Four commercial peroxides are included for comparison purposes. Of the acyl peroxycarbonic esters tested, the acetyl derivatives gave higher peak temperatures than the other three, indicating a somewhat higher degree of cure.

The data in Tables V and VI demonstrate the improved utility of APSC over two commercial peroxides presently used. In Table V, Hot Block gel tests were done at 140-200° F. using three peroxides. Results indicate that APSC "kicks-off" at a lower temperature than either 2,5-dimethyl-2,5-di(2-ethyl hexanoyl peroxy) hexane or benzoyl peroxide. In addition, lower concentrations of APSC can be used to achieve similar or superior results over the entire temperatue range.

Table VI shows that APSC can also be used in room temperature curing of unsaturated polyesters using the same promoter systems as used with benzoyl peroxide (BPO). Several tertiary amines commonly used in the industry were screened. Results indicate that faster cure cycles are obtained with the APSC systems as compared to the BPO systems. In addition, smaller amounts of promoter can be used with APSC to match the performance of the equivalent BPO system. Tertiary amines are known to be poor promoters for outdoor applications with BPO because of the yellowing problem. However, with APSC, the color is much lighter even at equivalent amounts of any of these amines.

The polyester resins used in the work reported in Tables IV-VI, Hatco GR 14017 and GR 941 are isophthalic polyesters containing styrene monomers.

                                      TABLE IV                                     __________________________________________________________________________     Hot Block Gel Tests With Polyester Resin Using Various Peroxides.sup.1)        Resin: Hatco GR941 (Orthophthalic)                                             Block Temp: 180° F. (82° C.)                                     Concentration of each peroxide equal to AO equivalent of 3.50%                                Ten-Hour Half-Life.sup.2)                                                                       Gel Time,                                                                            Exotherm                                                                             Peak Temp,                         Peroxide       Temp, ° C                                                                          % Wt Used                                                                            Min.  Time, Min.                                                                           ° F (° C)            __________________________________________________________________________       LPOC         51.4       0.82  2.87  3.97  260 (127)                            PPCC         51.5       0.78  2.65  3.67  272 (133)                            PPSC         52.3       0.85  2.72  3.79  272 (133)                            APSC         54.9       0.82  3.35  4.60  293 (145)                            APCC         55.1       0.75  3.36  4.58  289 (143)                            t-Butyl peroxy pivalate                                                                     55         0.38  3.05  4.00  298 (148)                            Decanoyl Peroxide                                                                           62         0.75  3.87  4.99  293 (145)                            2,5-Dimethyl-2,5-di-                                                                        68         0.47  6.3   7.63  303 (151)                            (2-ethyl hexanoyl peroxy)                                                      hexane                                                                         Benzoyl peroxide                                                                            72         0.53  8.35  10.76 264 (129)                          __________________________________________________________________________      +hu 1) 5 cc of a 50 g mixture charged to Hot Block                             .sup.2) 0.2 M. peroxide in benzene                                       

                                      TABLE V                                      __________________________________________________________________________     Hot Block Gel Tests at 140-200° F Using Three Peroxides.sup.1)          Resin: Hatco GR14017 (Isophthalic)                                                                           2,5-Dimethyl-2,5-                                                              di(2-ethyl hexanoyl                              Block Temp.,                                                                          Peroxide APSC          Peroxy) hexane                                                                             Benzoyl Peroxide                     ° F                                                                            Conc,% wt. 0.5 0.75                                                                               1   0.5   1     1                                    __________________________________________________________________________     140    Gel Time, Min                                                                             --  --  20.56                                                                              --    --    --                                          Exotherm Time, Min                                                                        --  --  22.43                                                                              --    --    --                                          Peak Temp, ° F                                                                     --  --  262.5                                                                              --    --    --                                   150               --  12.91                                                                              8.94                                                                               --    19.91 --                                                     --  14.10                                                                              10.33                                                                              --    21.35 --                                                     --  276 287.5                                                                              --    292.5 --                                   160               7.47                                                                               6.01                                                                               4.46                                                                               13.94 8.34  23.84                                                  8.86                                                                               7.04                                                                               5.31                                                                               15.34 9.47  25.26                                                  291.8                                                                              303 304 293.5 313.5 287                                  170               4.32                                                                               3.31                                                                               2.97                                                                               7.35  4.69  10.06                                                  5.31                                                                               4.09                                                                               3.60                                                                               8.50  5.52  11.25                                                  303.2                                                                              310.8                                                                              313 317   315   311                                  180               3.0 2.29                                                                               1.86                                                                               5.25  3.41  8.01                                                   3.66                                                                               2.97                                                                               2.43                                                                               5.97  3.85  8.84                                                   308.0                                                                              314 317 318   314   322.5                                200               0.86                                                                               0.53                                                                               0.40                                                                               1.34  0.74  2.33                                                   1.46                                                                               1.19                                                                               0.94                                                                               1.89  1.22  2.89                                                   315 317.3                                                                              318.5                                                                              324   328   326                                  __________________________________________________________________________      +HU 1) 5 cc of a 50 g. mixture charged to Hot Block                      

                  TABLE VI                                                         ______________________________________                                         Room Temperature Gel Tests Using 1% wt. Acetyl                                 Peroxy Stearyl Carbonate (APSC)                                                or Benzoyl Peroxide (BPO) With Various Promoters.sup.1)                        Temperature: 23-24.5° C.                                                Resin: 50 g Hatco GR-941                                                                                         Exo-                                                                   Gel     therm                                                          Conc,   Time,   Time    Peak                                 Peroxide                                                                              Protomer   % Wt.   Min.    Min.    Temp, ° C                     ______________________________________                                         1. APSC                                                                               (a) DMA.sup.2)                                                                            0.15    8.3     15.0    166                                                    0.30    4.15    10.0    169.5                                       (b) DEA.sup.3)                                                                            0.30    22.8    31.5    166                                         (c) DMT.sup.4)                                                                            0.15    4.35     9.5    168                                         (d) MPEA.sup.5)                                                                           0.15    16.25   23.0    172.2                                2. BPO (a) DMA    0.30    9.8     17.0    161.5                                       (b) DEA    0.30    38.1    47.0    166.5                                       (c) MPEA   0.30    13.7    20.5    166.6                                ______________________________________                                          +HU 1) 50g of mixture used in a 3oz. water cup                                 .sup.2) Dimethyl aniline                                                       .sup.3) Diethyl aniline                                                        .sup.4) Dimethyl p-toluidine                                                   .sup.5) Methyl phenyl ethanol amine                                      

Polymerization of MMA

Bulk polymerization of methyl methacrylate was done at 70° C. using six peroxides at equal molar concentrations. Results are shown in Table VII. t-Butyl peroxy privalate was included in this test because it is a commercial peroxide with a comparable ten-hour half-life temperature to the acyl peroxycarbonic esters. Except for LPOC, all of the initiators cured the monomer in about the same length of time. However, APSC and APCC were somewhat faster than the others.

The data in Table VII was obtained in the following manner.

The approximate amount of 100 percent of pure initiator required for 30 grams of monomer mixture was weighed into a 2-ounce wide-mouth bottle to 0.1 milligram. The amount of inhibited monomer needed for the desired initiator concentration was calculated and weighed to 0.01 gram. The bottle was capped and shaken, and 25 grams of the mixture was then weighed into a 25 × 150 millimeter test tube.

A stainless steel temperature-recorded probe was centered in the test tube through three rubber stoppers with the probe protruding a distance of approximately 70 millimeters. Before each run, the end of the probe was coated with a 3 percent aqueous solution of polyvinyl alcohol as a moldrelease agent.

The probe was inserted while flushing the test tube with a stream of nitrogen and the sealed unit placed into a stirred constant-temperature bath (70±0.2° C.) containing glycerine, to a depth of 110 millimeters.

                                      TABLE VII                                    __________________________________________________________________________     Polymerization of Methyl Methacrylate at 70° C With Various Acyl        Peroxycarbonic Esters                                                          1.72 × 10.sup.-4 mole initiator/100g MMA in 25 mm tube                                  Exotherm Time,                                                                             Peak Time,                                                                             Time to Return                                                                           Temp Rise, ° C             Peroxide %Wt Used                                                                             Min.        Min.    to Bath Temp, Min.                                                                       (ΔT)                        __________________________________________________________________________     1. APSC  0.064 108         113     142       53                                2. APCC  0.059 112         117     143       57                                3. PPSC  0.066 138         168     223       25                                4. PPCC  0.062 132         158     200       28                                5. LPOC  0.064 very long flat exotherm                                                                            ˜360.sup.1)                                                                        ≦5                         6. t-Butyl peroxy                                                                       0.03  135         143     180       51                                 pivalate                                                                      __________________________________________________________________________      +hu 1) Cured hard much slower than other runs shown.                     

Initiators for the Suspension Polymerization of Vinyl Chloride and Vinyl Chloride/Vinyl Acetate.

Suspension polymerization was done at 60° C. in pop bottles for both homopolymer and copolymer. Again, t-butyl peroxypivalate was included in the testing because it is used commercially for this purpose. As can be seen from the date in Table VIII, the acyl peroxycarbonic esters are not as efficient as t-butyl peroxypivalate on an equal weight basis, but are superior to it on an equal molar basis.

The data in Table VIII was obtained in the following manner.

The suspension polymerizations were performed in 12 fluid ounce pop bottles using uninhibited monomer. Each bottle contained 188.0 g. of suspension solution and 75 g. of monomer (water/monomer ratio = 2.5/1). The amount of suspension agent per 100 g. monomer = 0.35 g. Methocel 90HG, 100 cps. For the copolymer runs, 75 g. of a mixture of 88%wt vinyl chloride and 12%wt vinyl acetate were used.

After the desired amounts of pure initiator were added to the pop bottles, they were capped, the contents almost melted, and the bottles placed in a rotating constant temperature bath at 60° C. for varying lengths of time. Duplicate bottles were pulled at each time interval. The bottles were frozen before venting-off excess monomer. The polymer was filtered, washed, and dried.

                  TABLE VIII                                                       ______________________________________                                         Suspension Polymerization of Vinyl Chloride and                                Vinyl Chloride-Vinyl Acetate in Pop bottles at 60° C                                   % Conversion                                                                % Wt.    Time,                                                     Peroxide    Used     Hrs     1.5   3.5   5                                     ______________________________________                                         (A) Homopolymer                                                                1. APSC     0.055            9.9   32.5  53.3                                              0.118.sup.1)     21.1  60.5  91.8                                  2. APCC     0.055            10.4  34.7  58.1.sup.2)                                       0.109.sup.1)     20.5  65.4  93.2                                  3. t-Butyl peroxy                                                                          0.055.sup.1)     15.0  53.4  84.4                                  privalate (TBPP)                                                               (B) Copolymer                                                                  4. APSC     0.055            12.1  32.2.sup.2)                                                                          52.6                                              0.118.sup.1)     22.0.sup.2)                                                                          64.0  89.7                                  5. PPSC     0.055            12.2  --    52.9                                              0.122.sup.1)     23.3  60.8  86.0                                  6. TBPP     0.055.sup.1)     18.9  57.3  79.1                                  ______________________________________                                          +HU 1) 3.16 × 10.sup.-4 mole/100 g VCM or VC-VA                          .sup.2) poor duplication                                                 

Vulcanization of Silicone Rubber

Vulcanization of a general purpose silicone rubber was done in the Brabender Plasticorder at 110° C. using four initiators at equal molar concentrations. Results are shown in Table IX. 2,4-Dichloro benzoyl peroxide (2,4 DiCl BZP) was used in this work because it is the preferred commercial peroxide used for vulcanization of silicone rubber. Although the acyl peroxycarbonic esters are not as efficient as 2,4 DiCl BZP, they do vulcanize this type of silicone rubber. APSC and APCC gave almost identical results, whereas PPSC was only about 25 percent as efficient as they were.

The data in Table IX was obtained in the following manner.

53 g. of milled rubber (Dow Corning Silastic 4506U) were added within 1 minute to the mixing head maintained at the desired temperature. The rubber was added in small pieces. Net torque is equal to the maximum torque minus the minimum torque just before crosslinking begins. The time to reach maximum torque is equal to the elapsed time from zero.

                                      TABLE IX                                     __________________________________________________________________________     Vulcanizaton of Silicone Rubber in the Brabender Plasticorder                  with Various Acyl Peroxycarbonic Esters at 110° C                       1.645×10.sup.-3 mole initiator / 100g Silicone Rubber; 30 RPM Rotor      Speed                                                                            Peroxicde                                                                               phr 50% Paste                                                                          Net Torque, Mg.                                                                         Time to Reach Maximum Torque,                      __________________________________________________________________________                                 Min.                                               1. APSC    1.23    798      4.3                                                2. APCC    1.13    790      4.3                                                3. PPSC    1.27    208      5.4                                                4. 2,4-Dichlorobenzoyl                                                                    1.25    950      2.4                                                 Peroxide                                                                      __________________________________________________________________________ 

We claim:
 1. A process for the curing of polyester resins comprising subjecting ethylenically unsaturated polyester resin and cross-linkable monomer to cross-linking conditions in the presence of a cross-linking initiating amount of an acyl peroxycarbonic ester of the formula: ##STR4## where R is selected from methyl and ethyl; and R' is an alkyl group which together with R contains a total of 17 to about 25 carbon atoms.
 2. The process in accordance with claim 1, wherein R and R' contain a total of 17 to about 20 carbon atoms.
 3. The process in accordance with claim 1, wherein R is methyl.
 4. The process in accordance with claim 1, wherein R is ethyl.
 5. The process in accordance with claim 3, wherein R' is a cetyl group.
 6. The process in accordance with claim 3, wherein R' is a stearyl group.
 7. The process in accordance with claim 4, wherein R' is a cetyl group.
 8. The process in accordance with claim 4, wherein R' is a stearyl group.
 9. The process in accordance with claim 1, wherein the polymerization is carried out in the presence of a tertiary amine promoter. 